We have performed experiments designed to test the hypothesis that long-term depression (LTD) of excitatory synaptic transmission in the cerebellar cortex is caused by a rise in postsynaptic Ca concentration. These experiments combined measurements of synaptic efficacy, performed with the thin slice patch clamp technique, with fura-2 measurements of intracellular Ca concentration ([Ca]i) in single cerebellar Purkinje cells. Simultaneous activation of the climbing fiber and parallel fibers innervating single Purkinje cells caused a LTD of transmission of the parallel fiber-Purkinje cell excitatory synapse. This LTD was associated with large and transient rises in [Ca]i in the Purkinje cell and apparently was due to Ca entry through voltage-gated Ca channels in the Purkinje cell dendrites. The rise in [Ca]i produced by climbing fiber activity was necessary for LTD, because addition of the Ca chelator bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA) to the interior of the Purkinje cell blocked LTD. Further, elevation of [Ca]i, produced by depolarizing pulses delivered in conjunction with parallel fiber activation, induced a depression of synaptic activity that closely resembled LTD in both time course and magnitude. Thus, a rise in [Ca]i appears to be sufficient to initiate LTD. From these results, we conclude that LTD of the parallel fiber-Purkinje cell synapse is initiated by a brief, climbing fiber-mediated rise in postsynaptic [Ca]i and that LTD is maintained by other, longer-lived processes that are triggered by the rise in postsynaptic [Ca]i.